Open bennahugo opened 1 year ago
I've written some fitting software that I use to extract the ionospheric behaviour from the EVPA on the limb of a disk --- very similar to software Bryan / Eric worked on for AIPS workflow. This is here: https://github.com/bennahugo/lunaticpolarimetry/. We can use this to compare with results from ALBUS with different earth magnetic models.
Re the AIPS model: From my discussion with Eric and pulling AIPS TECOR Fortran apart..... the implementation is a simple dipole model inside AIPS at least AIPS MAGDIP.FOR shows the model dates back to the 1960s
SUBROUTINE MAGDIP (GLAT, GLONG, RADIUS, H)
C-----------------------------------------------------------------------
C! Calculate Earth's magnetic field components
C# Util
C-----------------------------------------------------------------------
C; Copyright (C) 1996
C; Associated Universities, Inc. Washington DC, USA.
C;
C; This program is free software; you can redistribute it and/or
C; modify it under the terms of the GNU General Public License as
C; published by the Free Software Foundation; either version 2 of
C; the License, or (at your option) any later version.
C;
C; This program is distributed in the hope that it will be useful,
C; but WITHOUT ANY WARRANTY; without even the implied warranty of
C; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
C; GNU General Public License for more details.
C;
C; You should have received a copy of the GNU General Public
C; License along with this program; if not, write to the Free
C; Software Foundation, Inc., 675 Massachusetts Ave, Cambridge,
C; MA 02139, USA.
C;
C; Correspondence concerning AIPS should be addressed as follows:
C; Internet email: aipsmail@nrao.edu.
C; Postal address: AIPS Project Office
C; National Radio Astronomy Observatory
C; 520 Edgemont Road
C; Charlottesville, VA 22903-2475 USA
C-----------------------------------------------------------------------
C-----------------------------------------------------------------------
C Routine to compute the earth's magnetic field strength from an
C offset dipole model. Adapted from Handbook of Geophysics and Space
C Envirnoments (circa 1965) S. L. Valley ed. Air Force Cambridge
C Research Laboratories and Chapman and Bartels, 1940, GEOPHYSICS,
C Oxford)
C NOTE: The Gaussian coefficients from Chapman and Bartels give
C a slightly better representation of the field than Valley so these
C values are used here.
C Values of H returned are probably good to better than 20%.
C At the VLA the model is 6% low in total intensity and 11 deg W in
C magnetic declination.
C Inputs:
C GLAT R Geocentric latitude (radians)
C GLONG R Geocentric EAST longitude (radians)
C RADIUS R Distance from the center of the earth (m)
C Output:
C H(3) R Magnetic field vector (gauss),
C (1) = positive away from earth center,
C (2) = positive east,
C (3) = positive north.
C-----------------------------------------------------------------------
REAL GLAT, GLONG, RADIUS, H(3),
* RE, FACT, GLATMP, GLONMP, PI,
* H02, L0, L1, L2, E, SQRT3,
* G10, G11, G20, G21, G22, H11, H21, H22, X0, Y0, Z0
PARAMETER (PI = 3.14159265)
C Geographic coordinates of
C North magnetic pole.
PARAMETER (GLATMP = 78.63 * PI / 180)
PARAMETER (GLONMP = 289.85 * PI / 180)
C Gaussian coefficients(gauss):
C From Handbook of Geophysics...
C Epoch 1960.
C Modified??????
C PARAMETER (G10 = -0.30509)
C PARAMETER (G11 = -0.02181/2.0)
C PARAMETER (G20 = -0.02196/2.0)
C PARAMETER (G21 = 0.05145/3.0)
C PARAMETER (G22 = 0.01448/4.0)
C PARAMETER (H11 = 0.05841/2.0)
C PARAMETER (H21 = -0.03443/3.0)
C PARAMETER (H22 = 0.00172/4.0)
C Chapman values Epoch 1922
PARAMETER (G10 = -.3095)
PARAMETER (G11 = -.0226)
PARAMETER (G20 = -.0067)
PARAMETER (G21 = 0.0292)
PARAMETER (G22 = 0.0143)
PARAMETER (H11 = 0.0592)
PARAMETER (H21 = -.0122)
PARAMETER (H22 = 0.0113)
C SQRT3 = sqrt (3.0)
PARAMETER (SQRT3 = 1.732050808)
C Compute dipole center in units
C of earth radius.
PARAMETER (H02 = G10*G10 + G11*G11 + H11*H11)
PARAMETER (L0 = 2.0*G10*G20 + (G11*G21 + H11*H21) * SQRT3)
PARAMETER (L1 = -G11*G20 + (G10*G21+G11*G22+H11*H22) * SQRT3)
PARAMETER (L2 = -H11*G20 + (G10*H21-H11*G22+G11*H22) * SQRT3)
PARAMETER (E = (L0*G10 + L1*G11 + L2*H11) / (4.0*H02))
PARAMETER (X0 = (L1 - G11*E) / (3.0*H02))
PARAMETER (Y0 = (L2 - H11*E) / (3.0*H02))
PARAMETER (Z0 = (L0 - G10*E) / (3.0*H02))
REAL X0M, Y0M, Z0M, HMAG, HD(3), CLA, SLA, CLO, SLO
DOUBLE PRECISION POS0(3), POS1(3), POSTMP(3), POST2(3), RADDIP,
* COLAT,
* LONDIP, CA, SA, CB, SB
C RE = Radius of earth (avg polar
C and equitorial)
DATA RE /6367650.0/
C-----------------------------------------------------------------------
C Center of dipole
X0M = X0 * RE
Y0M = Y0 * RE
Z0M = Z0 * RE
C Convert to earth center x,y,z
C Here y=> 90 e long.
POS0(1) = RADIUS * COS (GLAT) * COS (GLONG)
POS0(2) = RADIUS * COS (GLAT) * SIN (GLONG)
POS0(3) = RADIUS * SIN (GLAT)
C Translate
POSTMP(1) = POS0(1) - X0M
POSTMP(2) = POS0(2) - Y0M
POSTMP(3) = POS0(3) - Z0M
C Rotate to dipole coord.
CA = COS (GLONMP)
SA = SIN (GLONMP)
CB = SIN (GLATMP)
SB = -COS (GLATMP)
POST2(1) = (POSTMP(1)*CA + POSTMP(2)*SA) * CB +
* POSTMP(3) * SB
POST2(2) = POSTMP(2) * CA - POSTMP(1) * SA
POST2(3) = POSTMP(3) * CB - SB * (POSTMP(1)*CA + POSTMP(2)*SA)
C Polar coordinates in dipole.
RADDIP = SQRT (POST2(1)*POST2(1) + POST2(2)*POST2(2) +
* POST2(3)*POST2(3))
COLAT = ACOS (POST2(3) / RADDIP)
LONDIP = ATAN2 (POST2(2), POST2(1))
CLA = SIN (COLAT)
SLA = COS (COLAT)
CLO = COS (LONDIP)
SLO = SIN (LONDIP)
C Terms of dipole, local
FACT = SQRT (H02) * ((RE / RADDIP) ** 3)
H(1) = -2.0 * FACT * COS (COLAT)
H(2) = 0.0
H(3) = FACT * SIN (COLAT)
C Rotate to dipole centered
HD(1) = (H(1)*CLA - H(3)*SLA) * CLO - H(2) * SLO
HD(2) = H(2) * CLO + (H(1)*CLA - H(3)*SLA) * SLO
HD(3) = H(3) * CLA + H(1) * SLA
C Modulus of HD
HMAG = SQRT (HD(1)*HD(1) + HD(2)*HD(2) + HD(3)*HD(3))
C Find position 1 km from
C position in the direction of HD.
POST2(1) = POST2(1) + 1000.0 * HD(1) / HMAG
POST2(2) = POST2(2) + 1000.0 * HD(2) / HMAG
POST2(3) = POST2(3) + 1000.0 * HD(3) / HMAG
C Rotate new position to earth
C system.
POSTMP(1) = (POST2(1)*CB - POST2(3)*SB) * CA - POST2(2) * SA
POSTMP(2) = POST2(2) * CA + (POST2(1)*CB - POST2(3)*SB) * SA
POSTMP(3) = POST2(3) * CB + POST2(1) * SB
C Translate to earth center
POS1(1) = POSTMP(1) + X0M
POS1(2) = POSTMP(2) + Y0M
POS1(3) = POSTMP(3) + Z0M
C Earth centered field
HD(1) = (POS1(1) - POS0(1)) * 0.001 * HMAG
HD(2) = (POS1(2) - POS0(2)) * 0.001 * HMAG
HD(3) = (POS1(3) - POS0(3)) * 0.001 * HMAG
C Earth local field
CLA = COS (GLAT)
SLA = SIN (GLAT)
CLO = COS (GLONG)
SLO = SIN (GLONG)
H(1) = (HD(1)*CLO + HD(2)*SLO) * CLA + HD(3) * SLA
H(2) = HD(2) * CLO - HD(1) * SLO
H(3) = HD(3) * CLA - (HD(1)*CLO + HD(2)*SLO) * SLA
C
999 RETURN
END
It will be useful to translate this into / wrap it with some python so we can get apples to apples with what AIPS assumes, CASA assumes, RMextract assumes and ALBUS assumes.
You can use RMextract to compare the EMM magnetic field model vs the WMM model. The comments in the code above indicate the AIPS code dates from 1965 - we're still using that - really? The IGRF model gets updated about every 5 years or so. and WMM/EMM are I believe, at least as recent as 2015.
Some progress on this front - for the same time ranges (kludging the date in igrfv11 to accept 2021):
AIPS uses IONEX and ALBUS of course uses RINEX from stations close to site, so the difference is mainly in the measured TEC to source. I will try to next get the fortran MAGDIP.f bound into ALBUS to see what kind of order of effect this has on the resultant RM to quantify the errors
Just a note that there are now Python interfaces to the IGRF magnetic field model. Would be great to get away from having to shoehorn the IGRF Fortran into ALBUS
Yup we should - including f2c code is a bit ugly. There are also python-based RINEX conversion routines that I want to include instread of the proprietary binaries that we currently have to volume mount in.
Making a note to link this to an investigation of upgrading the current IGRFv12 inside casacore as mentioned on casacore/casacore#1114